Glass darkens upon exposure to ionizing radiation. When the glass is in the form of an optical fiber, the amount of light capable of being transmitted through the fiber varies as a function of the amount of ionizing radiation that has accumulated in the fiber. This principal has been proposed as a basis for the construction of a device for measuring radiation dosage. Optical fibers may be wound on a mandrel or around an open core to provide considerable length exposure to radiation. Such devices are discussed in U.S. Pat. No. 2,972,051 issued Feb. 14, 1961 to John W. Blum entitled "Pocket Radiation Dosimeter". and in NRL R&D HIGHLIGHTS, December 1974, entitled "New Real-Time Dosimeters Use Fiber Optics". By providing optical fibers of considerable length there is in effect an extension or elongation of the material which the ionizing radiation strikes, and which the transmitted light must traverse.
The impingement of ionizing radiation on the glass optical fiber results in alteration of the light absorbing properties as a function of the quality and quantity of radiation. By comparing light absorbed in the optical fiber before and after exposure, it is possible to measure the accumulated radiation dose. However, dosimeters relying upon the above principle have not been altogether satisfactory. The relation between fiber coloration or optical density falls off rapidly as soon as the radiation ceases due to fading of the coloration.
The most extensively studied effect of ionizing radiation on optical fibers has been signal attenuation due to production of defect centers which absorb the light transmitted through the fibers. Another important effect of radiation not so thoroughly studied is the luminescence generated in optical fiber waveguides during or immediately following exposure to radiation. The present invention relates to the use of the dose-dependent behavior of luminescence produced in silica core fibers during and after exposure to ionizing radiation.
The observation of intense luminescence at around 450 nm in fused silica at 4.2K immediately following irradiation with a pulse of 600 keV electrons is reported by George H. Sigel, Jr. in J. Non-Cryst. Solids, 13, 372 (1973/74). An emission centered at 450 nm from .alpha.-Quartz at 85K under gamma ray stimulation is reported in Radiation Effects, 26, 237 (1975). Luminescence at around 450 nm in Ge-doped optical fibers under electron irradiation is reported in IEEE, Trans. Nucli. Sci. NS-21, 113 (1974).
There is disclosed herein a new ionizing radiation induced emission band at 650 nm (in addition to the 450 nm band) which is characterized by a strikingly different dose-dependence of its intensity relative to the 450 nm intensity and how the two bands are employed for detecting immediate radiation and cummulative radiation.